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Abstract

We demonstrate a technique to continuously tune center frequency and repetition rate of optical frequency combs generated in silicon microring modulators and bandwidth scale them. We utilize a drive frequency dependent, microwave power induced shifting of the microring modulator resonance. In this work, we demonstrate center frequency tunability of frequency combs generated in silicon microring modulators over a wide range (∼8nm) with fixed number of lines. We also demonstrate continuously tunable repetition rates from 7.5GHz to 15GHz. Further, we use this effect to demonstrate a proof-of-principle experiment to bandwidth scale an 8-line (20dB band) comb generated from a single ring modulator driven at 10GHz to a comb with 12 and 15 lines by cascading two and three ring modulators, respectively. This is accomplished by merging widely spaced ring modulator resonances to a common location, thus coupling light simultaneously into multiple cascaded ring modulators.

Figures (6)

Fig. 1. a) A wide span superluminescent diode (SLED) at low power is used to characterize the effect of microwave power on a silicon microring modulator (OSA: optical spectrum analyzer). (b) Transmission spectra showing the effect of microwave power swept from 10 mW to 320 mW, in steps of 20 mW on the ring modulator resonance (c) The ring-modulator resonance undergoes a microwave power induced thermo-optic shift of ∼8 nm at 320 mW of source power (from 1547.56 nm to 1555.52 nm), inset: microscope image of the silicon microring modulator (d) deterioration in Q-factor by a factor of ∼3.

Fig. 3. Microwave power induced thermo-optic tuning of ring modulator resonances allows for wide center frequency tuneability of several nm. Here shown with combs driven at different repetition rates (a) 7.5GHz (9 lines) (b) 10GHz (8 lines) (c) 12.5GHz (7 lines) and (d) 15GHz (6 lines). (e) The tuning range of the center frequency for the repetition rates used in (a) to (d) as a function of applied microwave power. Data is acquired in steps of 0.5dB and in each case, the comb maintains the same number of lines.

Fig. 4. (a) Technique to bandwidth scale frequency combs generated through cascading of silicon microring modulators. Here a laser at λ0 is coupled simultaneously to two ring modulators that are optically coupled to a common bus waveguide. The microring modulators are driven with microwave power to align and tune their respective resonances to a common location λ using microwave power induced thermo-optic shift. (b) Setup used for bandwidth scaled frequency comb generation by cascading two ring modulators. Two microring modulators with radius of 7.68um and 7.59um have initial resonance locations at 1549.86 nm and 1550.61 nm respectively. The SLED source is used to trace the movement of the ring modulator resonances.

Fig. 5. a) Cascaded Ring modulator bank resonances with (no RF-power) containing 4 rings with resonance locations at 1546.91 nm, 1547.92 nm, 1549.86 nm, 1550.61 nm coupled to a common bus waveguide b) RF-power at 10 GHz applied to two ring modulator resonances leading to microwave power induced thermo-optic shifts. c) The RF-power applied to the ring modulators is tuned to merge the resonances to a common location (∼1554.72 nm). d) Optimized 10 GHz repetition rate combs generated from cascading two ring modulators with 12 lines in a 20 dB band with a center frequency of 1553.82 nm. e) Cascading three ring modulators using a different set of merged resonances resulting in 15 lines in a 20 dB band with a center wavelength of 1549.05 nm.